An oscilloscope is a type of electronic test instrument that allows observation of time-varying electrical signals. During typical operation, an oscilloscope receives an input signal through an oscilloscope probe connected to a device under test (DUT) and displays the received signal on an electronic display.
In certain contexts, it may be desirable to use an oscilloscope to observe signals over a large range of values, i.e., over a high dynamic range. For instance, when characterizing a mobile phone, it may be desirable to observe its signal characteristics when operating at a low current state, such as a sleep state, and when operating at a high current state, e.g., a signal transmission state. Moreover, it may also be desirable to observe signals at varying levels of scope or resolution, e.g., at a zoomed-in level and a zoomed-out level.
When using an oscilloscope to observe signals over a high dynamic range, it is not uncommon for the signals to exceed the normal operating range of various oscilloscope components, such as its input amplifier and/or analog to digital converter (ADC). As an example, when performing signal integrity measurements (e.g., overshoot, undershoot, ripple), a user may want to measure small aberrations on the top and bottom of the signals. To observe these aberrations, the user may increase the effective resolution and accuracy of the oscilloscope display by offsetting the input signal and then increasing the vertical sensitivity around a waveform portion of interest. This will spread out the small aberrations of the signal over a larger range of the oscilloscope's ADC. This technique will generally improve the resolution of the measurement on the aberrations, but it may also drive major portions of the signal off-screen and beyond the dynamic range of the oscilloscope's input amplifier and ADC.
Where an input signal exceeds the dynamic range of the above or other components, it may saturate the output of those components or engage overdrive protection circuitry until the input signal returns to within the dynamic range. Thereafter, the oscilloscope probe output may exhibit distortion during a period of “overdrive recovery” in which the components return to normal operation. For instance, if an input signal saturates the input amplifier and then subsequently returns to within the amplifier's linear operating range, the amplifier's output may exhibit nonlinear distortions during a period after the input signal returns to the linear operating range.
Due to these distortions and other factors, it is generally undesirable to allow oscilloscope input signals to exceed the oscilloscope's dynamic range. Conventionally, if an oscilloscope probe's output exceeds the oscilloscope's dynamic range, the recommended solution is to increase the volts per division (V/div) of a corresponding oscilloscope channel until the full screen voltage is greater than the output voltage. However, increasing the V/div of an oscilloscope channel increases channel noise and reduces ADC resolution, thus hindering the ability to discern small signals. For oscilloscope probes with large dynamic range outputs, the overdrive recovery of the oscilloscope input does not allow the signal to be viewed accurately at maximum sensitivities. Large signals cannot be observed at sub-millivolt accuracies due to channel input noise and lack of ADC resolution.
In view of the above and other shortcomings of conventional approaches, there is a general need for techniques and technologies to accurately view small voltages in a large dynamic range probe output.